FIG. 1 is a schematic illustrating a conventional structure of an LTE system supporting RN. As shown in FIG. 1, in an access network of an LTE system, wireless resource management entities may include macro base stations (eNB) 101 and relay nodes (RN) 102. An RN accesses a core network via another type of macro base stations (DeNB) 103. The eNBs 101 are connected with each other via X2 interfaces. Each of the eNBs 101 are connected with mobility management entities (MME) and serving gateways (S-GW) 104 in the core network via S1 interfaces. The RNs 102 access the DeNBs 103 via Un interfaces. A DeNB 103 provides X2 proxy functions between RNs 102 and other eNBs. A DeNB 103 provides S1 proxy functions between RNs 102 and MME/S-GWs 104. S1 proxy function and X2 proxy functions include transmission of UE-dedicated X2 signaling and S1 signaling between an RN 102 and an eNB 101 and between an RN 102 and an MME 104, and transmission between an RN 102 and an S-GW 104.
FIG. 2 is a schematic illustrating a conventional protocol stack of an S1 interface on a control plane supporting relay. There is an S1 interface between an RN and a DeNB of the RN, and there is an S1 interface between a DeNB and each MME in an MME pool. A DeNB processes and forwards all UE-dedicated S1 signaling between an RN and an MME. The processing of UE-dedicated S1 messages by a DeNB includes modifying S1-application protocol UE identifications (S1-AP UE IDs), transport layer addresses and GTP TEIDs, and keeping other parts of the messages unchanged.
Conventional relays are deployed at fixed locations, and do not support mobility across different cells. At present, a problem faced by operators is that in high-speed trains, e.g. a train traveling at a speed of 250-350 km/h, current relays can not provide satisfactory service quality due to factors such as big noises, high penetration loss, severe Doppler frequency shifts, low handover success ratio, and so on. Operators thus start researches in mobile relays. Mobile relays are proposed to eliminate the deficiencies of conventional relays, to improve service quality provided in high-speed trains, and to serve users better. In the current system framework, a DeNB stores UE context information of UEs served by an RN. The DeNB needs to allocate UE S1 AP IDs, a TEID and a transport layer address for each of the UEs served by the RN. In a high-speed running train, there are a lot of users who are relatively static from the perspective of the train. When an RN moves from one DeNB to another DeNB, it is desirable to transfer context of a UE from the serving DeNB to the target DeNB and the target DeNB needs to allocate UE S1 AP IDs, a TEID and a transport layer address for the UE, which adds complexity to the moving process. In addition, a mobile RN is required to support multiple access techniques, e.g. 2G/3G/LTE, but the Un interface supports the LTE access technique only. A DeNB implements proxy functions according to the above framework of a fixed RN. If a UE accesses a 3G network, a DeNB is required to proxy an Iu message and to support an Iu protocol, which is complicated for a DeNB. An optimized RN framework may solve the above problems, e.g. the control plane merely transparently transports S2 signaling between an RN and an MME and signaling of other access techniques between an access network and a core network. Therefore, a DeNB does not have to store context information of a UE or allocate resources to the UE. During movements of an RN, if a UE is relatively static and the MME serving the UE is unchanged, the movements of the RN are transparent for the UE. A problem of the above framework exists in how to transparently transport S1 signaling between an RN and an MME via a DeNB because the DeNB can not identify the destination node using information in an S1 message when the DeNB does not parse the S1 message. One solution to the problem is to interact via the transport layer about how to route an S1 message. The problem of the above manner of interacting via the transport layer about how to route an S1 message is how an MME or an RN is able to get knowledge about transport layer information and/or information of an access layer of each other.
The above takes an interface between a UE and an RN according to the LTE access technique as an example. The above problem still exists when a UE accesses other types of core networks by using other access techniques. For example, the problem of how to transport information of the transport layer and information of the access layer also exists in the process of establishing an interface and communication between an RN and an SGSN in a 3G network and between an RN and an MSC in a 2G network.